(a) Field of the Invention
The present invention relates to an inkjet printer and method of printing, and more in detail to the inkjet printer and the method of printing in which high quality printing can be conducted by increasing the number of gray scales of printing density.
(b) Description of the Related Art
A non-impact recording method which is excellent in its negligibly small noise at printing is attracting public interest. An inkjet recording technology included in the non-impact recording method enables high speed recording on a recording medium in a simple mechanism and conveniently employs an ordinary plain paper as the recording medium.
The inkjet recording systems are roughly categorized into a continuous jet system and an on-demand system (impulse system). Since the on-demand system is driven depending on a necessity to eject ink droplets, ink consumption is moderate. The structure thereof is also extremely simple. Accordingly, wide spread of the on-demand system is expected.
A conventional inkjet printer using the on-demand system is described in JP-A-59(1984)-198162, JP-A-58(1983)-39468 etc. in which the following examples are mentioned. A first conventional example is such that dots are printed after they are converted into a specified matrix size for conducting medium tone recording (dither method). A second conventional example includes a plurality of ink chambers accommodating inks having different densities, a plurality of nozzles for each of the ink chambers and a plurality of dot forming section corresponding to at least two inks of the same color having different densities. In the latter printer, gray scale level of a pixel formed as a matrix is generated by changing the number of ink particles supplied into the matrix and the density of the ink particles in accordance with a gray scale signal. A third example is such that the size of ejected ink droplets is changed by modifying conditions of a driving pulse of a piezoelectric device.
Ordinarily, in order to conduct full-color printing (16.77 million colors), 256 gray scale of 256 levels for each color of Y (yellow), M (magenta) and C (cyan) are required. When the gray scale of 256 levels per dot is realized in a specific area of for example, 600 dots.times.800 dots of a CRT screen, the amount of information reaches to 600 dots.times.800 dots.times.3 bite=1.44 MB because the 256 levels are expressed as 1 bite (=2.sup.8).
In this text, an ink having a higher density is defined as "normal ink" and an ink having a lower density is defined as "light ink". A binary level in which a dot of the normal ink having a specified dot size is implemented by "printed" or "not printed" in the first conventional example. The amount of information in this case reaches only to 11.25 kB (600 dots.times.800 dots.times.3.times.2 (binary value)/256). In order to output the 256 levels, a dither matrix of 16 dots.times.16 dots(=256) is necessary.
A ternary level in which a dot of the normal ink having a specified dot size is "printed", or a dot of the light ink having a specified dot size is "printed" or none of the inks are "printed" is utilized in the second conventional example. Assuming that a relative printing density of the normal ink is defined as "1", that of the light ink is defined as "1/2" and that of a printing paper is defined as "0" in the second conventional example, the amount of information reaches to 600 dots.times.800 dots.times.3.times.3(ternary value)/256=16.875 kB. When the light ink takes charge of 128 levels from 0 to 127 and the normal ink takes charge of 128 levels from 128 to 255, a dither matrix of 12 dots.times.12 dots (.apprxeq.128) is necessary to output 256 levels.
In the third conventional example, a 4 level gray scale is utilized in which a dot size of an ink is variable and a large droplet is "printed", a medium droplet is "printed", a small droplet is "printed" and none of the droplets are "printed". Assuming that a relative printing density of the normal ink having the large droplet is defined as "1", that having the medium droplet is defined as "2/3", that having the small droplet is defined as "1/3" and that having none of the droplets is defined as "0" in the third conventional example, the amount of information reaches to 600 dots.times.800 dots.times.3.times.(four-value)/ 256=22.5 kB. Assuming that the small droplet takes charge of 86 levels from 0 to 85, the medium droplet takes charge of 85 levels from 86 to 170 and the large droplet takes charge of 85 levels from 171 to 255, a dither matrix of 9 dots.times.9 dots (.apprxeq.85) is necessary to output 256 levels.
In the first conventional example, a matrix of 256 dots or "16.times.16" is required to output 256 levels, and the amount of information with respect to all the gray scale levels is as small as to 11.25 kB. In the binary level employing only the relative densities "0" and "1", coarseness of the image quality appears in a highlighted area to generate a low quality picture.
Although, in the second conventional example, the amount of information with respect to all the inks is 16.875 kB, the quality of the picture which is elevated 1.5 times that of the first example is not yet satisfactory. In the ternary level employing the relative densities "0", "0.5" and "1", the coarseness of the quality as well although the image quality is somewhat improved over the first example.
In the third conventional example, a limit of the variable range of the dot size exists wherein the small dot is difficult to print, and only the relative printing densities "0" (no dots are printed) and "about 0.3 to 1" can be realized. Although the region between 0.3 and 1 may be more finely divided in principle, this division has little influence on the improvement of the picture quality, and the division of the range may be at the most three levels. Accordingly, the 4 level gray scale per dot is appropriate as described herein, the amount of information increases to 22.5 kB, and a dither matrix required for outputting a 256 level gray scale is reduced to 9.times.9 dots (.apprxeq.85). Although the picture quality of the third example is better that those of the first and the second examples, it is not satisfactory. The coarseness of the picture is still noticeable because the limit of the variable range of the dot size exists and the small dot cannot be printed.
An approach for improving the picture quality such as multi-level processing and improvement of resolution (dpi: dot per inch) is known, but the present inventor intends to obtain a higher quality picture by employing further high multi-level processing to make the present invention based on an original idea.